The present invention relates generally to systems incorporating “motor power units” including motor soft starters and motor drives as are used to control industrial motors, and in particular to an interface circuit that simplifies the assembly of systems incorporating motor power units.
The sophisticated control of large electrical motors used in industry and manufacturing may be provided through the use of an electronic “motor drive” or “motor controller” (herein collectively termed “motor power units”) having solid-state power processing circuitry controlled by microprocessor circuitry receiving control signals to start and stop the motor while providing overload and for motor drives, short circuit protection functions. An example advanced feature implemented by motor power units is a “soft start” where power is applied gradually to the motor after a start command is received to reduce surge currents and mechanical shock to attached equipment. Overload and short circuit protection functions, when provided, may not only disconnect the motor but also may provide current limiting and power management functions.
In practice, motor power units are normally incorporated into a metal control cabinet used to hold and interconnect the motor power unit and other circuit elements such as circuit breakers and disconnect switches. The control cabinet may have an enclosing front panel serving as mounting point for panel controls (such as switches and indicator lights) providing an interface with the motor power unit and associated circuitry.
The panel controls may include one or more electrical switches used to provide signals to the motor power unit to start and stop the motor being controlled. Such panel controls may be implemented in a number of different forms. In a “two-wire” system, a multiposition switch connects to the motor controller to “start” the motor in a first position and “stop” the motor in a second position. In a “three-wire” system, two independent momentary-contact pushbuttons may be used. A “start” pushbutton starts the motor when it is pressed. The motor remains running when the start pushbutton is released until a “stop” pushbutton is pressed.
Variations on the two-wire system includes a three-state “hand/off/auto” system which allows the incorporation of remote control to the motor (auto) as well as a manual run state (hand) and stop state (off). Another variation may combine two-wire and three-wire control.
Motor power units typically include separate “start” and “stop” input terminals to receive commands from the panel controls. The different control systems (two-wire, three-wire, etc.) may be implemented by using different panel controls and changing the wiring pattern between the panel controls and the motor power unit. Different control configurations can require many different wiring patterns increasing the complexity of manufacture and the possibility of error in the assembly of control cabinets.